The invention relates to a display tube comprising an evacuated envelope having a display window and containing means to generate at least one electron beam which is deflected over a display screen provided on the inside of the display window. The display screen comprises a luminescent layer on which a thin electron-permeable aluminium film is provided. The display screen also includes carbon particles.
The invention also relates to methods of manufacturing a display screen for such a display tube.
The aluminium film in such a display tube effects an increase in the brightness of the picture. The film operates as a mirror which reflects a part of the light generated in the luminescent layer through the display window in the direction of the viewer.
Such display tubes may be tubes for monochromatic display of pictures, for example black-and-white television display tubes, projection television display tubes, cathode ray tubes as used in oscilloscopes and tubes for displaying letters, digits and characters (so-called D.G.D. tubes, where D.G.D. represents Data Graphic Display).
The luminescent layer in D.G.D. tubes often consists of material luminescing in one colour. However, such tubes may also be tubes for displaying coloured pictures. In case that the luminescent layer of the display screen often consists of a large number of triplets of elements luminescing in three different colours, sometimes separated by lightabsorbing material. By using a colour selection electrode in the tube, each of the three electron beams generated in the tube is associated with luminescent elements of a respective colour. The most frequently used colour selection electrode is the shadow mask.
Such a display tube is known from Netherlands Patent Application No. 6800398 laid open to public inspection corresponding to British Pat. No. 1,152,290 in which a colour display tube having post-acceleration and post-focusing is described. A porous carbon layer is provided on the thin aluminium film to absorb the greater part of the secondary and reflected electrons which occur in such a post-acceleration tube. Netherlands Patent Application No. 6,916,046, corresponding to British Pat. No. 1,260,079 discloses a colour display tube in which a layer of graphite (carbon) is used on the aluminium film to absorb the thermal radiation originating from the colour selection electrode. The electron beams in a colour tube impinge on the colour selection electrode or the display screen and produce thermal energy. Inter alia because more electrons impinge on the colour selection electrode, it becomes warmer than the screen. In order to prevent the thermal energy radiated from the colour selection electrode to the display screen from being reflected by the aluminium film to the colour selection electrode, the aluminium film bears a heat-absorbing carbon layer.
In the manufacture of display tubes it has been found that the aluminium film on which a porous heatabsorbing and/or secondary and reflected electronsabsorbing porous layer of carbon particles was provided, corroded in a moist atmosphere. The corrosion of the aluminium occurs at particular in those places where the aluminium film is in contact with the carbon particles where an electrochemical reaction occurs between water and aluminium. The most important factor is the relative humidity of the atmosphere. At a relative humidity of 80% or higher the corrosion of the aluminium film is so large that measures have to be taken to protect the aluminium film.
Such corrosion of the aluminium film also occurs in colour display tubes in which light absorbing material consisting mainly of carbon particles was provided between the luminescent elements.